Trypanosoma cruzi: reversal of inhibition of host muscle differentiation after exposure to elevated temperatures.

The L6E9 myoblast cell line can be grown as individual cells in "growth medium," or can be induced to fuse and differentiate to form multinucleated myotubes either at 37 C or at 40.5 C in "differentiation medium." It has previously been shown that myoblasts with infected Trypanosoma cruzi (Brazil strain) cannot differentiate to form myotubes. Moreover, the mRNAs for contractile proteins are not induced in these infected cells. Infected myoblasts grown in "differentiation medium" at 37C were unable to differentiate by 7 days. The infection was maintained at 100%, and the number of trypomastigotes in the supernatant increased with time (peak greater than 10(6)/ml). At 40.5C, however, infected myoblasts gradually eliminated their infection. The percentage of parasitized cells was reduced to less than 1% by the 7th day of observation. There was also a decrease in the number of trypomastigotes in the supernatant. Moreover, significant fusion was observed in these cultures by morphological criteria. Using 32P-labeled recombinant DNA probes, it was shown that, at 37C, there was an inhibition of mRNAs for muscle-specific contractile proteins (myosin heavy chain and alpha-actin), whereas nonspecific mRNAs were not inhibited. Furthermore, infected myoblasts exposed to 40.5C exhibited no inhibition of mRNAs for myosin heavy chain and alpha-actin. Myoblasts cleared of their infection could readily be reinfected. This study demonstrates that the inhibition of muscle differentiation induced by T. cruzi is reversible when cultures are exposed to elevated temperatures.